Allow building buildx from source

Signed-off-by: CrazyMax <crazy-max@users.noreply.github.com>

src/git.ts

@@ -0,0 +1,19 @@
+import * as exec from '@actions/exec';
+
+export async function getRemoteSha(repo: string, ref: string): Promise<string> {
+  return await exec
+    .getExecOutput(`git`, ['ls-remote', repo, ref], {
+      ignoreReturnCode: true,
+      silent: true
+    })
+    .then(res => {
+      if (res.stderr.length > 0 && res.exitCode != 0) {
+        throw new Error(res.stderr);
+      }
+      const [rsha, rref] = res.stdout.trim().split(/[\s\t]/);
+      if (rsha.length == 0) {
+        throw new Error(`Cannot find remote ref for ${repo}#${ref}`);
+      }
+      return rsha;
+    });
+}